Ah sorry, forgot the traceback and command:

Command: python3 Lib/test/test_argparse.py 
test (__main__.TestActionRegistration) ... ok
test_failures_many_groups_listargs (__main__.TestActionUserDefined) ... ok
test_failures_many_groups_sysargs (__main__.TestActionUserDefined) ... ok
...
...
======================================================================
FAIL: test_help_with_metavar (__main__.TestWrappingMetavar)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_argparse.py", line 5152, in test_help_with_metavar
    '''))
AssertionError: 'usag[55 chars]_name [-h] [--proxy <http[s]://example:1234>]\[113 chars]4>\n' != 'usag[55 chars]_name\n       [-h] [--proxy <http[s]://example[121 chars]4>\n'
- usage: this_is_spammy_prog_with_a_long_name_sorry_about_the_name [-h] [--proxy <http[s]://example:1234>]
?                                                                 ----------------------------------------
+ usage: this_is_spammy_prog_with_a_long_name_sorry_about_the_name
+        [-h] [--proxy <http[s]://example:1234>]
  
  optional arguments:
    -h, --help            show this help message and exit
    --proxy <http[s]://example:1234>


----------------------------------------------------------------------

Ran 1574 tests in 4.558s

FAILED (failures=1)
Traceback (most recent call last):
  File "Lib/test/test_argparse.py", line 5164, in <module>
    test_main()
  File "Lib/test/test_argparse.py", line 5156, in test_main
    support.run_unittest(__name__)
  File "/usr/local/lib/python3.7/test/support/__init__.py", line 1991, in run_unittest
    _run_suite(suite)
  File "/usr/local/lib/python3.7/test/support/__init__.py", line 1910, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "Lib/test/test_argparse.py", line 5152, in test_help_with_metavar
    '''))
AssertionError: 'usag[55 chars]_name [-h] [--proxy <http[s]://example:1234>]\[113 chars]4>\n' != 'usag[55 chars]_name\n       [-h] [--proxy <http[s]://example[121 chars]4>\n'
- usage: this_is_spammy_prog_with_a_long_name_sorry_about_the_name [-h] [--proxy <http[s]://example:1234>]
?                                                                 ----------------------------------------
+ usage: this_is_spammy_prog_with_a_long_name_sorry_about_the_name
+        [-h] [--proxy <http[s]://example:1234>]
  
  optional arguments:
    -h, --help            show this help message and exit
    --proxy <http[s]://example:1234>



I first encountered this error when running "make test" when installing python 3.7.2, then tried running this test manually with the command above.